As a means of obtaining more efficient immunotherapy of tumors, we are investigating possible methods for increasing the cytotoxicity of antibodies. Antitumor antibodies have been conjugated to various toxic agents such as radioactive iodine, diphtheria toxin and nitrogen mustards. The toxicity of the conjugates is being evaluated with tumor cells in tissue culture. We are also investigating the cytotoxicity of antibody-enzyme conjugates with the expectation that we can utilize the catalytic activity of the enzyme to convert a protoxin substrate to a toxin. Toxin- or enzyme-antibody conjugates which appear promising in vitro will be investigated in vivo in experimental animals. We are studying the effects of antibodies on tumor cell metabolism and growth both in vivo and in vitro. BIBLIOGRAPHIC REFERENCES: M.F. Fink, C.W. Parker and W.T. Shearer: "Antibody stimulation of tumor growth in T-cell depleted mice." Nature 255:404, 1975. Shearer, W.T., Philpott, G.W. and Parker, C.W.: "Humoral immunostimulation. II. Increased nucleoside incorporation, DNA synthesis, and cell growth of L cells treated with anti-L cell antibody." Cellular Immunology 17:447, 1975.